Designing generator systems

When designing generator systems, such as engine-driven generator sets, design engineers must ensure that the generator selection, sizing, and the design of electrical systems are appropriate for the application.

Tarek G. Tousson, PE, Stanley Consultants, Austin, Texas

03/14/2016

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Learning objectives

Explain the basics of designing generator systems.

Outline the impact of load types on generator system design.

Analyze the codes and standards that pertain to designing a generator system.

When designing generator systems, engineers must ensure that the generators and the building electrical systems they support are appropriate for the application. Many questions must be addressed before starting the generator system design. Most of these questions are related to the application and the site operating conditions, which drives the selection of the different generator system components and their characteristics.

The answers to these questions will help engineers make decisions regarding generator sizing, loading sequence, paralleling, fuel storage, switching scenarios, and many other criteria.

Article 445, Generators, contains installation and other requirements for generators, such as location, marking, overcurrent protection, ampacity of conductors, and others.

Article 700, Emergency Systems, applies to those systems legally required to automatically supply illumination and power essential for safety to human life in the event of exiting places of assembly during normal power failure or an accident. Per 700.12, General Requirements, when normal power is lost, emergency system power is required to be applied in 10 seconds or less.

Article 701, Legally Required Standby Systems, applies to those standby systems legally required by municipal, state, federal, or other codes, or by any governmental agency having jurisdiction. These systems are intended to automatically supply power to selected loads (other than those classed as emergency systems) that could create hazards or hamper rescue or firefighting operations when stopped in the event of failure of the normal power source. Per 701.12, General Requirements, when normal power is lost, standby system power is required to be applied in 60 seconds or less.

Article 702, Optional Standby Systems, applies to those systems intended to provide an alternate supply (onsite-generated power) to selected loads, either automatically or manually, where interruptions to the electrical system cause serious discontinuation of operation.

Article 705, Interconnected Electric Power Production Sources, applies to synchronous generator sets that operate in parallel.

Article 708, Critical Operations Power Systems (COPS), applies to those systems required to continuously operate for reasons of public safety, emergency management, or national security. These systems are intended to automatically supply power to COPS loads in the event of failure of the normal power source.

Additional codes and regulations apply in some special applications such as fire pumps. NFPA 20-2016: Installation of Stationary Pumps for Fire Protection and NEC Article 695, Fire Pumps, provide guidelines for the generator system design. Per 695.7 [20:9.4], the voltage shall not drop more than 15% of the controller's normal voltage at the fire pump controller under motor-starting conditions.

Early involvement of the permitting agency and the authority having jurisdiction will help the design engineer determine which set of codes and regulations apply to the generator system during the design phase. It will also help the design engineer understand the different interpretations of the codes.

Site operating conditions

The location of the generator system to be installed will have substantial impact on how the system is built and arranged. If the generator system is to be installed indoors in a designated room or co-located with other building system equipment, an open-type generator-set configuration can be used (see Figures 1 and 2). However, other concerns may arise. Generator sets and ancillary equipment must be accessible for operation and maintenance, building load-bearing capacity must be adequate to house the generator set(s) and ancillary equipment, construction must comply with applicable codes and regulations (noise, emissions, vibration, etc.), and room layout must satisfy manufacturer requirements for adequate combustion-air intake, fuel supply, ventilation, and exhaust. All building system equipment co-located with the generator system must be evaluated because it must be able to operate in the same environmental conditions.

If the generator system is to be installed outdoors, the generator must be provided with a weatherproof enclosure for protection from environmental conditions, such as rain, snow, corrosion, flooding, etc. The enclosure also will restrict undesired access and protect the generator from possible vandalism. Additional features, such as acoustical lining and silencer grade, may need to be specified when selecting the weatherproof enclosure to meet the permitted noise level. A walk-in weatherproof enclosure is convenient for performing maintenance in severe weather conditions. A double-wall subbase tank is an added feature to provide diesel fuel storage. Portable or trailer-mounted generator systems are required in cases where the generator cannot be permanently installed onsite or needs to be used for backup power at multiple locations.

Load characteristics

Load types and electrical characteristics of loads to be served by the generator system have a significant impact on the generator system size, system configuration topology, and complexity of designing the system. Design engineers must understand the load profile and assess the different design options to provide a reliable and economical generator system with a rating based on load profile and run time requirements.

Load types are linear and nonlinear. Linear loads include heaters, motors, and transformers. Nonlinear loads include computers, uninterruptable power supplies (UPSs), electronic lighting ballasts, electronic equipment, and variable frequency drives. Nonlinear loads typically introduce harmonics in the electrical system. Depending on the magnitude of harmonics and total harmonic distortion present in the electrical system, the generator set will be derated to reliably support the loads. Linear loads, such as large motors, have a high starting current (locked-rotor current) that requires the generator set to be oversized in some cases to overcome the demand.

The size of the load relative to the size of the generator system also can have significant impact on the operation of the generator system. Whenever a load is applied to or removed from a generator set, the engine speed, voltage, and frequency will experience a transient condition or a temporary change from its steady-state condition. If the connected load or load-block pickup is too large, the generator may not be able to start. The engine speed, voltage, and frequency may drift outside of the generator system operating limits and trip offline, or they may deviate beyond limits acceptable to the load and cause the load to trip offline. Thus, load characteristics and generator system response capabilities are important design parameters that must be considered.

After defining the load characteristics involved, a load profile must be developed describing the sequence of supplying generator power to the different load types and increments, and how often those loads are switched on and off.